Field of the Invention
The invention relates to a data transmission circuit having a station and a response circuit. The station has an amplitude demodulator and a primary coil with a signal generator for producing an alternating magnetic field at a carrier frequency. The response circuit has a secondary coil as well as an amplitude modulator for influencing the load on the secondary coil, and the amplitude modulator is configured such that the alternating magnetic field can be modulated with a data signal.
Data transmission circuits of this generic type are used, in particular, in simultaneous powering and reading (SPR) systems where the power and data are transmitted inductively.
Such SPR systems are also used in applications with non-contact smart cards.
During use, the signal generator in the station produces a periodic signal on the primary coil, which results in an alternating inductive field or an alternating magnetic field being formed in its vicinity, which field acts as a so-called "close field" in an area around the primary coil. In contrast to an electromagnetic wave originating from the primary coil, the purely inductive effect of the signal emerging from the primary coil is the primary factor in the close field of the primary coil.
A response circuit which draws its operating power, in particular, from the alternating magnetic field can be moved into the area of the close field. For this purpose, the response circuit is equipped with a secondary coil, in which the alternating magnetic field induces an alternating voltage. The alternating voltage induced there is rectified and smoothed in the response circuit, and is supplied to a data signal production block. The data signal production block is connected to an amplitude modulator (which is disposed, in particular, in the vicinity of the secondary coil) in such a manner that it can vary the load on the secondary coil as a function of a data signal produced by the data signal production block.
To this end, it is known from the prior art for the amplitude modulator to be configured as a variable resistive load, with the resistance load on the secondary coil being varied in a suitable manner in accordance with the data signal. Such a change in the resistance load on the secondary coil results in the electrical characteristics of the primary coil at the station end also varying, since the primary coil and the secondary coil are inductively coupled. The coupling factor of the inductive coupling is, as a rule, between one percent and five percent.
The alternating magnetic field can be modulated with the data signal from the response circuit in the manner mentioned above when the response circuit is in the area close to the primary coil.
On the primary coil side, the amplitude demodulator samples the voltage dropped across the primary coil, which voltage varies with the data signal, and uses this to reconstruct the data signal.
The data transmission circuit of this generic type allows response circuits to be supplied with power in a reliable manner, while furthermore ensuring that a data signal transmitted by the response circuit can be read at the station end.
However, when data transmission circuits of this generic type are used in practice, it has been found that, particularly when the data transmission circuit is mass-produced, situations frequently occur in which the data signal modulated onto the alternating magnetic field by the response circuit can no longer be reconstructed at the station end. Particularly when a transmission circuit of this generic type is used on immobilizers intended for motor vehicles, this has led to the user of a motor vehicle not being able to use his motor vehicle, despite authorization.
Published, British Patent Application GB-A-2 232 851 discloses a loose-coupled transformer via which a measurement circuit, for example on a moving part of a vehicle, is supplied with power. The transformer is loaded by the measurement electronics via a switch, to be precise at a frequency which is an integer multiple of the power supply frequency. The periodic load is phase-modulated when the level of a binary data signal, which is transmitted by the measurement circuit, varies. The modulated signal is transferred back to the primary of the transformer, and is multiplied by a periodic signal in order to demodulate it for data recovery.